This invention pertains to an inverter generation apparatus for converting an output of a DC power circuit to generate a DC voltage from an output obtained from a power supply source of a generator into an AC voltage having a desired waveform by using an inverter.
In many cases, there have been used an inverter generation apparatus for converting a DC voltage obtained by rectifying an output of an AC generator into an AC voltage having a predetermined peak value and a predetermined frequency by using an inverter as an electric power supply apparatus including the AC generator driven by a primer such as an internal combustion engine and so on.
Such an inverter generation apparatus comprises a DC power circuit to output a constant DC voltage a bridge inverter to convert the DC voltage from the DC power circuit into an AC voltage, a controller to control the inverter, a filter to remove a harmonic component out of the output of the inverter, load connection terminals across which an output of the filter is applied, a load current detection circuit to detect a load current and an output voltage detection circuit to detect a voltage across the load connection terminals.
The DC power circuit comprises an internal combustion engine having a rotational speed controlled so as to rotate at a constant speed, a three-phase AC generator driven by the internal combustion engine, a control rectifier circuit to rectify an output of the AC generator, a control circuit to control the control rectifier circuit so as to maintain a DC voltage from the control rectifier circuit at a constant value and a smoothing capacitor connected across output terminals of the control rectifier circuit, for example.
The control rectifier circuit comprises a mixed bridge circuit having thyristors and diodes connected in the form of bridge so as to form a three phase full wave rectifier circuit. Three phase input terminals of the three phase full wave rectifier circuit are connected to three phase output terminals of the generator and the smoothing capacitor is connected to DC output terminals of the three phase full wave rectifier circuit.
The bridge inverter comprises a conventional circuit including four switch elements connected to each other in the form of bridge. The bridge inverter serves to convert the output voltage of the DC power circuit into the AC voltage by controlling the respective switch elements of the inverter so that a period during which one pair of switch elements located at one dialogical position of the bridge are in the on-state and a period during which another pair of switch elements located at another dialogical position thereof are in the on-state alternately appear.
The filter provided on the output side of the inverter is formed of a low pass filter comprising coils, capacitors and resistors and serves to remove a harmonic component included in the output of the inverter to form the output
The controller serves to generate a pulse width modulation signal (PWM signal) commanding to turn on and off the predetermined switch elements of the inverter at a predetermined duty ratio to each of the switch timings appearing for a predetermined PWM period so as to generate the AC voltage of a given waveform across the pair of the load connection terminals.
Also, the controller serves to read the instant value of the voltage across the load connection terminals through the output voltage detection circuit and compare the actually read instant value of the AC voltage with the desired instant value of the AC voltage to be obtained across the load connection terminals, which is previously stored in a ROM or the like of the microcomputer to correct the on-off duty ratio of the switch elements of the inverter to each of the switch timings. This obtains the AC output voltage of sine wave having the desired instant value across the load connection terminals.
Furthermore, the controller serves to stop supplying a drive signal to each of the switch elements of the inverter when the load current exceeds the allowable maximum value and then the value of the output voltage of the load current detection circuit exceeds the maximum value. Thus, the controller stops the operation of the inverter and prevents the switch elements of the inverter from being damaged due to its overcurrent.
However, in case that the AC voltage of the desired value is obtained across the load connection terminals by feeding back the instant value of the AC voltage across the load connection terminals to the controller to correct the duty ratio so that the AC voltage of desired magnitude is obtained across the load connection terminals, the waveform of the AC output voltage is disadvantageously distorted when the output voltage of the DC power circuit is lower than the peak value of the AC output voltage to be obtained across the load connection terminals and when the output voltage of the DC power circuit exceeds the peak value of the AC output voltage to be obtained across the load connection terminals.
Thus, in the prior inverter generation apparatus, it is difficult to maintain the waveform of the AC output voltage in case that the output voltage of the DC power circuit has the narrow allowance range of variation allowing the desired waveform of the AC output voltage to be maintained and therefore in case that the load has the larger variation width.
Accordingly, it is a principal object of the invention to provide an inverter generation apparatus adapted to have a wider allowance range of variation of an output voltage of a DC power circuit allowing a waveform of an AC output voltage without any distort of the waveform of the AC output voltage even though the output voltage of the DC power circuit has some variation.
In accordance with one aspect of the invention, there is provided an inverter generation apparatus comprising a DC power circuit to obtain a DC voltage from an output of a generator as a power source, a bridge type inverter to which the DC voltage is input from the DC power circuit, a filter to remove a harmonic component of an output of the inverter, load connection terminals to which the output of the filter is input and a controller to control an on-off operation of each of predetermined switch elements of the inverter in a predetermined duty ratio to switch timings appearing for predetermined PWM period so as to obtain an AC output voltage of desired waveform across the load connection terminals, the controller comprising duty correction means to correct the duty ratio of the on-off operation of the element switches on-off driven to each of the switch timings for maintaining a predetermined waveform of the AC output voltage obtained across the load connection terminals even though the output voltage of said DC power circuit varies.
In a preferred mode of the invention, the inverter generation apparatus further comprises a DC voltage detection circuit to detect a DC voltage output by the DC power circuit and the controller comprises duty correction means to correct the duty ratio of said on-off operation of the element switches on-off driven to each of the switch timings in accordance with a magnitude of the DC voltage detected by the DC voltage detection circuit for maintaining a set peak value of the AC output voltage obtained across the load connection terminals.
In another preferred mode of the invention, the inverter generation apparatus further comprises a DC voltage detection circuit to detect a DC voltage VD output by the DC power circuit and the controller comprises reference duty ratio arithmetical operation means to arithmetically operate as a reference duty ratio Do an on-off duty ratio of the switch elements to each of the switch timings for forming a desired waveform of the AC output voltage obtained from the inverter through the filter, correction coefficient arithmetical operation means to arithmetically operate a correction coefficient Kv=VA/VD by which the reference duty ratio to each of the switch timings should be multiplied so that a peak value of the AC output voltage gets equal to a set value VA, real duty ratio arithmetical operation means to arithmetically operate an on-off real duty ratio D of the switch elements to each of the switch timings by multiplying the reference duty ratio Do by the correction coefficient Kv and drive signal supply means to apply a drive signal to each of the switch elements of the inverter so as to make an on-off operation of the switch elements in the real duty ratio obtained by the real duty ratio arithmetical operation means.
With the apparatus constructed as aforementioned, since the on-off duty ratio of the switch elements of the inverter can be corrected in accordance with the output voltage of the DC current power circuit, the allowable range of variation of the output voltage of the DC power circuit by which the sine wave form of the AC output voltage can be maintained can be wider than that of the prior art.
In a further preferred mode of the invention, the controller comprises reference duty ratio arithmetical operation means to arithmetically operate as a reference duty ratio Do an on-off duty ratio of the switch elements to each of the switch timings for forming a desired waveform of the AC output voltage obtained from the inverter through the filter, load voltage detection circuit means to detect the AC output voltage obtained across the load connection terminals; correction coefficient arithmetical operation means to arithmetically operate a correction coefficient Kv=VA/VD by which the reference duty ratio to each of the switch timings should be multiplied so that a value reflecting a cumulative value of the output voltage detected by the load voltage detection circuit during a period of 1/xcex1 or xcex1 (xcex1 is a positive integer of one or more than one) of the AC voltage obtained across the load connection terminals is made equal to a set value; real duty ratio arithmetical operation means to arithmetically operate an on-off real duty ratio D of the switch elements to each of the switch timings by multiplying the reference duty ratio Do by the correction coefficient Kv and drive signal supply means to apply a drive signal to each of the switch elements of the inverter so as to make an on-off operation of the switch elements in the real duty ratio D obtained by the real duty ratio arithmetical operation means.
There may be used an average value of the AC voltage obtained from the cumulative value, an effective value thereof or a square of the effective value thereof, for example for the value reflecting the cumulative value. Otherwise, the value reflecting the cumulative value may be the cumulative value itself.
The value obtained by sequentially cumulating the output voltage detected by the load voltage detection circuit to each of the switch timings for the period better in view of the waveform of the AC voltage obtained across the load connection terminals, which may be a quarter of cycle, a half of cycle, one cycle, two cycles or three cycles of the AC voltage may be the value reflecting the variation in the output voltage of the DC power circuit.
Thus, as the duty ratio of the switch elements on-off driven to each of the switch timings is obtained by multiplying the reference duty ratio Do by the required correction coefficient Kv arithmetically operated so that the value such as the cumulative value, the average value, the effective value, the squared effective value and so on reflecting the cumulative value of the output voltage detected by the load voltage detection circuit to each sample timings for the period better in view of the waveform of the AC voltage obtained across the load connection terminals, the AC voltage having the predetermined peak value can be obtained across the load connection terminals without detecting the output voltage of the DC power circuit. This can omit the DC voltage detection circuit so that the apparatus has a simplified construction and reduce the cost thereof.
Also, with the apparatus constructed as aforementioned, since the duty ratio of the on-off operation of the switch elements of the inverter circuit can be controlled so that the value reflecting the cumulative value Of the detection value of the output voltage always gets the predetermined value in spite of the magnitude of the load current, large difference between the output voltage such as the average value or the effective value when loaded and the output voltage when unloaded can be effectively prevented.